1. Field of the Invention
This invention relates to electroacoustic apparatus, particularly to broadband microwave frequency delay devices.
2. Description of the Prior Art
Broadband microwave delay devices have been implemented with coaxial cable where the length of the cable determined the time delay. Present cables have resulted in about 30 db signal loss at 5.2 gigahertz for a delay of 100 nanoseconds. To obtain a flat frequency response from a cable delay line from 2.6 to 5.2 gigahertz, an attenuating filter must be placed in series with the cable to provide an approximate 1/f dependence in attenuation. The attenuating filter provides maximum attenutation at 2.6 gigahertz which diminishes as a function of 1/f to a nominal value at 5.2 gigahertz. Consequently, the coaxial cable delay line with the attenuating filter exhibits an insertion loss which is slightly greater than the maximum coaxial cable attenuation at the upper band edge.
Microwave bulk acoustic delay devices have been developed which are narrowband but tunable over a broadband. One narrowband microwave bulk acoustic delay device consists of a resonant cavity with a transducer at one end coupled to an acoustic transmission medium. The cavity can be tuned over a broadband with a tuning stub, but at any one time, the resonant cavity is tuned to a narrow band. Another implementation of a narrowband microwave bulk acoustic delay device has been made by using a transmission line to connect to a thin film transducer attached to a transmission medium. Because of the low impedance of the thin film transducers, a lumped inductor was placed adjacent to the transducer to increase the impedance to that of the transmission line. The lumped inductor impedance changes with frequency and the impedance characteristics limited the delay device to use over a narrow band.
A further problem with microwave bulk acoustic delay devices is that the acoustic propagation loss is proportional to the frequency squared, and therefore, over a broad band the frequency response due to propagation loss is not constant.
Microwave bulk acoustic delay devices are required to exhibit good electrical isolation between the input and the output. Electromagnetic coupling between the input and output transducers of a delay device or between the input and output connectors to the microwave delay device is the principal cause for loss of isolation.
In some acoustic delay devices, the acoustic signal received at the second transducer is also reflected back into the acoustic transmission medium, and subsequent reflections of this acoustic signal are returned back to the output transducer causing unwanted output signals. One example of unwanted output signals which occur when the transducers are at either end of the transmission medium path is called the triple transit signal. In this case, the signal is reflected from the second transducer to the first transducer and then back to the second transducer. In a typical microwave delay device, the triple transit energy is desired to be down approximately 40 db from the first single transit energy. In some microwave delay devices, because of the high triple transit energy, the insertion loss is intentionally increased or additional gating is providing to further attenuate the triple transit signal and subsequent reflected signals.